Visible light communication method and apparatus

ABSTRACT

A method and an apparatus are provided for guaranteeing terminal mobility in a time division visible light communication system. In resource assignment in a position where the visible light communication terminal changes a cell, the terminal is assigned a resource to be used in a next cell in a cell boundary area from a communication coordinator, thereby guaranteeing the continuous mobility. The visible light communication system informs the terminal of cell boundary notice information for easily informing the terminal of the cell definition so that the terminal can recognize that the terminal is currently located in the cell boundary and is necessary to change the resource.

PRIORITY

This application claims priority under 35 U.S.C. §119(a) to KoreanPatent Applications filed in the Korean Intellectual Property Office onOct. 31, 2009 and Jan. 18, 2010 and assigned Serial Nos. 10-2009-0104700and 10-2010-0004553, the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a visible light communicationmethod and apparatus, and more particularly to a method and an apparatusfor visible light communication, which can secure continuous serviceduring movement between cells of a visible light communication serviceusing a Time Division Multiplexing (TDM) scheme.

2. Description of the Related Art

Due to improvements in light emitting efficiency and decreases in pricefor Light Emitting Diodes (LEDs), the LED has been generally introducedinto the market of general lighting, which includes the florescent lampand the incandescence lamp, as well as the market of special lighting,which may include a portable device, a display, an automobile, a trafficlight, and an advertising board. Further, due to recent trends, such asthe exhaustion of frequencies in the Radio Frequency (RF) band,crosstalk probability between several wireless communicationtechnologies, increased demand for communication security, and theadvent of an ultra high-speed ubiquitous communication environment of 4Gwireless technology, interest in optical wireless technology compatiblewith RF technology has increased.

Visible light communication, which transfers information using visiblelight, is advantageous in that it can accurately recognize a receptionrange of information because it is possible to view a destination oflight or a progress direction of light. Visible light communication isalso safe, has a broad use band, and can be used without restriction.Therefore, visible light communication is reliable in a security aspectand can be driven with low power, which is advantageous in a powerconsumption aspect. In this respect, visible light communication can beapplied to a hospital and an airplane where the use of the RF band isrestricted, and also can provide additional information service using anelectric signboard. Visible light communication is described withreference to the drawings below.

FIG. 1 is a diagram illustrating a visible light communication systemusing a general Visible Light Communication (VLC). The general VLCsystem includes a light source 10 functioning as a lighting formed withan LED or a Laser Diode (LD), and transmitting/receiving data usingvisible light. The general VLC system also includes a VLC terminal 20including a visible light receiving/transmitting module, which performsdata transmission/reception with the light 10. The VLC terminal 20includes a mobile terminal, such as a mobile phone or a PDA, or a fixedterminal, such as a desktop computer. Further, the VLC can be linkedwith a communication system using another wired/wireless communicationmedium, so that it can be used more efficiently.

When it is necessary to provide service using the VLC to a broad space,the multiple light sources 10 are installed within a corresponding spaceaccording to a service range of each of the light sources 10. Forexample, as illustrated in FIG. 2, the multiple light sources 10 can beinstalled in a ceiling of a single room. Each of the light sources 10can be installed in a lattice-type pattern with a predeterminedinterval, considering a service area within which the single lightsource 10 can provide the light communication service.

As illustrated in FIG. 2, when the multiple light sources in a cell unitare installed with the interval in the general VLC system, the actualservice areas of each of the light sources 10 are overlapped, boundariesof the service areas are in contact with each other, or a non-servicearea is present between service areas of each of the light sources. Theideal state is when the boundaries of the service areas are in contactwith each other, but it is difficult to actually realize this state. Inthe general service areas according to the multiple light sources, anoverlapped area in which the service areas are partially overlappedexists or a non-service area exists between two service areas.

However, when the multiple light sources having the partially overlappedservice area provide different kinds of services, data transmitted bythe two services collide in the overlapped region, making it impossibleto provide the regular service.

In order to provide the regular service in the overlapped area,different time slot resources are used in adjacent cells, and theadjacent cells avoid the overlapped portion in time during thecommunication, thereby reducing inter-cell interference.

When a cell size increases in the VLC, the interference between theadjacent cells or the support of the terminal mobility is effective, butthe entire efficiency of the system is deteriorated. Further, when acell size decreases, the efficiency of the system is improved, but theinter-cell hand-off and the interference between adjacent cellsincrease.

Further, when the continuous mobility is decreased, the continuousmobility can be secured in using the time resource which is the same asthe time slot resource used in a previous cell. However, when anotheruser uses the time resource, which has been used in the previous cell,in a mobile cell, it is difficult to support the continuity.

SUMMARY OF THE INVENTION

The present invention has been made to address at least the aboveproblems and/or disadvantages and to provide at least the advantagesdescribed below. Accordingly, an aspect of the present inventionprovides a method and an apparatus capable of continuously providing theterminal mobility in the VLC service.

Another aspect of the present invention provides a method and anapparatus, which can reduce inter-cell interference and support theefficient mobility of a system, without changing a cell size, in the VLCservice.

According to an aspect of the present invention, a visible lightcommunication method is provided in a visible light coordinator, whichdiscriminates between at least one communication area cell including atleast one light source and a boundary area positioned between thecommunication area cell and another communication area cell adjacent tothe communication cell area and which provides time division visiblelight communication. A position of a visible light communicationterminal is detected. A first communication area cell corresponding tothe position of the detected visible light communication terminal isdetermined. A first time slot for receiving data of the visible lightcommunication terminal in the first communication area cell isdetermined. When the visible light communication terminal is located ina boundary area of the first communication area cell, a second timeslot, which has not been assigned to the visible light communicationterminal, is assigned to the visible light communication terminal. Thevisible light communication terminal is communicated with by using thesecond time slot.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following detailed descriptionwhen taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the construction of a general VLCsystem;

FIG. 2 is a diagram illustrating an example of a VLC system in whichmultiple light sources are arranged;

FIG. 3 is a block diagram illustrating a VLC terminal, according to anembodiment of the present invention;

FIG. 4 is a block diagram illustrating the construction of thecommunication control device, according to an embodiment of the presentinvention;

FIG. 5 is a diagram illustrating a cell pattern, according to anembodiment of the present invention;

FIG. 6 is a diagram illustrating a cell structure, according to anembodiment of the present invention;

FIGS. 7-10 are diagrams illustrating movement of a terminal betweencells, according to an embodiment of the present invention;

FIG. 11 is a diagram illustrating a frame structure, according to anembodiment of the present invention;

FIGS. 12-15 are diagrams illustrating movement of a terminal betweenadjacent cells, according to an embodiment of the present invention;

FIGS. 16A and 16B are flowcharts illustrating an operation of a VLCterminal, according to an embodiment of the present invention;

FIGS. 17A and 17B are flowcharts illustrating an operation of a basestation, according to an embodiment of the present invention; and

FIGS. 18 and 19 are diagrams illustrating movement of a terminal betweenadjacent cells, according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

Embodiments of the present invention are described in detail withreference to the accompanying drawings. In the following description,the same or similar elements may be designated by the same or similarreference numerals although they are shown in different drawings.Detailed descriptions of constructions or processes known in the art maybe omitted to avoid unnecessarily obscuring the subject matter of thepresent invention.

A VLC system according to an embodiment of the present inventiontransmits data by using a time division multiplexing scheme.Specifically, the VLC system transmits multiple pieces of data afterdividing the data according to uniform time slots, so as to configure asingle cell including a single light source or multiple light sources,thereby multiplexing the data with multiple channels.

The light source according to the present invention is a set of devicesincluding at least one light emitting device and at least onephotosensitive device. The VLC assigns a time slot resource for each ofthe users, each of the user services, or each of the contents of theVLC, so that it efficiently and variously transmits data throughmultiple channels. The VLC system includes a cell including at least onelight source, and discriminates and provides the user service in a unitof the cell. The single cell can provide at least one user service. Theuser service transmits any kind of data to the VLC terminal, and can beclassified based on a kind of content to be transmitted, a datatransmission scheme including broadcasting or multicasting, or a user.All of the light sources included in the single cell transmit identicaldata by using a specific downlink time slot resource. Therefore, in theevent that the light sources are included in the identical cell, evenwhen the light sources are different from each other, the light sourcescan transmit the identical data.

The assignment of the time slot resource in the VLC system, according toan embodiment of the present invention, can be dynamically changedaccording to a characteristic of the user service or a communicationenvironment during the provision of the identical user service. Further,the light sources and the number of light sources included in the singlecell can be dynamically changed according to a characteristic of theuser service or a communication environment during the provision of theidentical user service. Further, the light sources included in thesingle cell can be physically adjacent or not-adjacent to each other.The cell is not fixed and is formed by a logical mapping for the lightsources, not by a physical position of each of the light sources.

The light sources may be connected with a single base station ormultiple base stations. When the light sources are connected with themultiple base stations, the VLC system includes a controller forcontrolling the connection between the light sources and the multiplebase stations.

A system for providing the VLC service supporting mobility in a timedivision VLC system, according to an embodiment of the presentinvention, includes at least one light source, which transmits datareceived from at least one VLC terminal located in its service area ofthe VLC system to a communication coordinator and transmits datareceived from the communication coordinator to at least one VLCterminal. The system for providing the VLC service, according to anembodiment of the present invention, includes a VLC system including acontroller and a terminal. The controller makes a control such that atleast one light source to be included in a single cell according to thegenerated user service from one or more light sources is determined andgrouped, and the grouped cells into the single cell is mapped toconstitute a cell by assigning an identical ID or different IDs to thelight sources included in each cell. A predetermined time slot or atleast one time slot is assigned to each cell. The time slot to be usedfor providing the generated user service is assigned to the cell. Datarelated to the generated user service is transmitted to a correspondingVLC terminal through the light source mapped to the cell by using theassigned time slot resource. In order to support mobility of theterminal, at least one time slot is assigned to a signal transmittedfrom the light source to the terminal, which is used for the purpose ofdata service and movement. According to the movement of the terminalfrom the cell to an adjacent cell, a Photo Detector (PD) and the lightsource including the PC within each cell, in which the terminal moves,detects a terminal signal and identifies a position of the movingterminal within the cell by using ACK, NACK, and other response messagestransmitted from the terminal after the data reception. When theterminal approaches a cell boundary between the current cell and anadjacent cell, the PD and the light source informs the terminal of afact that the terminal is located in a cell boundary through boundarynotice information transmitted from the light source positioned at aboundary area of each cell. The time slot to be used in the adjacentcell, to which the terminal will move, is assigned to the terminal whenthe boundary notice information is transmitted. The time slot to be usedin the cell to which the terminal will move from the cell boundary isassigned and the assigned time slot is used in the next cell formovement of the terminal. The identical time slot is used in the entirecell boundaries in the VLC system. Different time slots are used in eachcell. The time slot to be used in the cell to which the terminal willmove is assigned in the cell boundary. The time slot to be used in thecell to which the terminal will move is assigned in the currentcommunication cell. Each base station is controlled when there aremultiple base stations.

An example of the VLC terminal in the VLC system, according to anembodiment of the present invention, is illustrated in FIG. 3. FIG. 3illustrates the construction of the VLC terminal.

Referring to FIG. 3, a VLC terminal 100 includes a first memory 111, acontroller 112, a first encoder 113, a first modulator 114, a firsttransmission driver 115, an LED 116, a selector 117, a first decoder118, a first demodulator 119, a first reception driver 120, and a PD121.

The controller 112 processes data for transmission/reception dataaccording to the VLC and controls the first encoder 113, the firstdecoder 118, the first transmission driver 115, and the first receptiondriver 120, to control the general operation of the VLC terminal 100 asdescribed below according to an embodiment of the preset invention.

The first encoder 113 encodes transmission data inputted from thecontroller 112 and outputs the encoded data to the first modulator 114.The first modulator 114 modulates the inputted transmission data andoutputs the modulated data to the first transmission driver 115.

The first transmission driver 115 serving as a driver of the LED 116optical-modulates the transmission data inputted from the firstmodulator 114 and drives the LED 116.

The LED 116 is a light emitting device provided for the transmission ofthe transmission data to an external apparatus by using an opticalsignal and is driven by the first transmission driver 115. The VLCterminal 100 can include various kinds of light emitting devices, andeach of the light emitting devices has a different supportablewavelength band according to its characteristic.

The PD 121 is a photosensitive device for detecting an optical signaltransmitted from the external apparatus. The PD 121 receives an opticalsignal including reception data from the light source and converts thereceived optical signal to an electric signal, to output the convertedelectric signal to the first reception driver 120. The VLC terminal 100can include various kinds of photosensitive devices, and each of thephotosensitive devices has different sensible wavelength band accordingto its characteristic. Generally, the supportable wavelength band of thelight emitting device included in the single VLC terminal 100 is similarto that of the photosensitive device included in the single VLC terminal100, and thus a kind of wavelength channels usable by the light emittingdevice and the photosensitive device may be identical to each other.

The first reception driver 120 is a driver for the PD 121, and adjusts awavelength detection band of the PD 121 according to a wavelength bandcorresponding to a wavelength channel selected by the selector 117.Further, the first reception driver 120 outputs the electric signalinput from the PD 121 to the first demodulator 119.

The first demodulator 119 demodulates the electric signal input from thefirst reception driver 120 to data according to an optical wirelesscommunication scheme, and outputs the demodulated reception data to thefirst decoder 118.

The first decoder 118 decodes the input reception data and outputs thedecoded data to the controller 112. The controller 112 appropriatelyprocesses the reception data input from the first decoder 118.

The first memory 111 stores a program for processing and controlling ofthe controller 112, reference data, various renewable storing data,etc., and is implemented as a working memory of the controller 112.

The construction of a communication control device, according to anembodiment of the present invention, will be described with reference toFIG. 4. An access point, the base station, or a base station accesspoint present in the following description includes the communicationcontrol device of FIG. 4. Therefore, the communication control devicecorresponds to the access point, the base station, or the base stationaccess point, and they can be treated as identical entities. FIG. 4illustrates the construction of the communication control device,according to an embodiment of the present invention. The light sourceconnected to the communication control device includes at least onelight source functioning as the transmission unit and at least onephotosensitive device functioning as the reception unit, the lightemitting device may include the LED, etc., and the photosensitive devicemay include a PD. The photosensitive device and the light emittingdevice included in the light source is operated and managed under thecontrol of the communication control device, so that they are physicallyclassified, but they can be connected with an electric power line or adata cable or perform the wireless communication.

Referring to FIG. 4, the communication control device, i.e., the accesspoint, includes a second memory 251, a communication controller 252, asecond encoder 253, a second modulator 254, a second decoder 225, asecond demodulator 256, and a first input/output end 259.

The second encoder 253, the second modulator 254, the second decoder255, and the second demodulator 256 operate in a manner similar to thatof the first encoder 113, the first modulator 114, the first decoder118, and the first demodulator 119, and serve similar respectivefunctions.

The first input/output end 259 transmits/receives data between thecommunication control device and the light source, and may include apower line, a wire data cable, or a wireless transmission/receptiondevice.

The second memory 251 stores a program for processing and controlling ofthe communication controller 252, reference data, various renewablestoring data, etc., and is implemented as a working memory of thecontroller 252.

The communication controller 252 processes data fortransmission/reception data according to the VLC, controls the secondencoder 253 and the second first decoder 255, and controls the generaloperation of the communication control device according to the presetinvention. Specifically, the communication controller 252 controls everyoperation necessary for cell mapping and management and the time slotresource change assignment and management, and may control a drivingdriver of each light source and photosensitive device according to timeslot resource change assignment information corresponding to each of theVLC terminals and light sources.

FIG. 5 illustrates a combination structure of the light emitting device(LED/LD) functioning as the transmission unit and the photosensitivedevice, e.g. the PD, functioning as the reception unit in thecommunication control device, i.e. the access point, in which the lightemitting device and the photosensitive device constitutes the lightsource applied to the VLC system.

Further, FIG. 5 illustrates several schemes for assigning a cell ID toeach light source. The light emitting device and the PD may be includedin a single module according to a manufacturing method. To helpunderstanding of the description, it is assumed that the light emittingdevice is the LED and the photosensitive device is the PD.

In FIG. 5, a construction cell n ID_n, in which a cell 301 includes asingle light source including a first LED 303 functioning as thetransmission unit and a PD 305 functioning as the reception unit and thesingle light source has an independent ID number ID_n within the singlecell Cell n, is illustrated. Specifically, an identical cell ID isassigned to the LED 303 and the PD 305.

A cell 307 has the construction in which the multiple light sourceshaving the structure of the cell 301 are combined within the single cellCell n to constitute a single cell. The transmission unit and thereception unit have a cell ID number, respectively, like a light source309, a PD 311, and a PD 311 within the cell 307, so that the LED and thePD performing the transmission and reception with the terminal duringthe communication with the terminal is discriminated using the cell IDnumber. Further, for terminal mobility provided in the presentinvention, by using the cell ID number of the LED and the PD within thesingle cell, the terminal can be aware of a position of the light sourceand the PD communicated within the cell. A cell 315 includes a singlelight source having an LED 317 and a PD 319. The LED 317 and the PD 319have respective cell ID numbers. The cell 315 is illustrated for thepurpose of describing the different transmission and reception functionsof the LED and the PD of the cell 315 from that of the cell 301. In acell 321, every light source included in the single cell Cell n has anidentical ID number ID_n. A cell 323 includes the single light sourceincluding multiple LEDs 325 functioning as the transmission unit and asingle PD 327 functioning as the reception unit. The multiple LEDs 325are bound such that they can operate like the single LED, and themultiple LEDs 325 and the single PD 327 are assigned an identical cellID number Cell n ID_n. Further, in order to improve a receptionefficiency, the light source of the cell can have a constructionincluding the single LED and the multiple PDs according to theconstruction. In a cell 329, the single light source includes multipleLEDs 331 and a single PD 333. Similarly to the light source of the cell315, a single cell ID number Cell n ID_n is assigned to the multipleLEDs 331 and a different cell ID number Cell n ID_n+1 is assigned to thePD 333. The light source of the cell 329 can have a constructionincluding the single LED and the multiple PDs according to theconstruction.

In a cell 335, a plurality of light sources included in the cell 329 arebound, for example light source 337 and 339, to form the single cellCell n, and each of the light sources are assigned an ID number. In acell 341 multiple cells including a different ratio between the numberof LEDs and the number of PDs constitutes a single cell. Theconstruction method of the cell is varies according to the constructionmethod of the VLC system, but the terminal mobility method suggested inthe present invention can be variously applied regardless of theconstruction of the light source and the PD.

To help understanding of the description, an embodiment of the presentinvention is described assuming that the single cell ID number isassigned to the light source in which the single LED is bound with thesingle PD. FIG. 6 illustrates the construction of the VLC system,according to the embodiment of the present invention. As illustrated inFIG. 6, in the VLC system, the multiple light sources are bound anddefined as a cell 401, a cell 403, and a cell 405. The multiple lightsources included in each cell are assigned a cell ID, to be assigned asthe single cell, and the cells are connected with a base stationapparatus (or Access Point (AP) 407. Further, each of the cells isassigned in a unit of the time slot resource, like a VLC frame 409, andeach cell is used for assigning the time slot resource serving as acommunicable time resource to the terminal included in each cell. Aterminal U1 included in the cell 401 of FIG. 6 is assigned a virtualtime (vs) slot n (vs n) serving as the time slot resource of the VLCframe 409. A terminal U2 included in the cell 403 of FIG. 6 is assigneda vs n+1 serving as the time slot resource of the VLC frame 409. Aterminal U3 included in the cell 405 of FIG. 6 is assigned a vs n+nserving as the time slot resource of the VLC frame 409. Therefore, it ispossible to avoid interference resulting from simultaneous communicationuse between the adjacent cells.

The frame 409 is divided into a downlink (D/L) and an uplink (U/L), sothat the terminal included in each cell uses the D/L and the U/L in theunit of the time slot resource assigned to the terminal. A scheduling ofthe time resource assignment is performed in the AP 407. C and M of theframe 409 of FIG. 6 refer to contention information and managementinformation, respectively. C and M have scheduling information of use ofcell resource information so that the scheduling information is commonlytransmitted to every terminal within the cell. Therefore, the terminalswithin the cell can identify the time slot resources assigned to theterminals by using the scheduling information.

In FIG. 6, when the VLC system assigns the time resources, the VLCsystem can assign the fixed time slot resource to each cell, to providethe user terminals within the cell with the service, such as thebroadcasting. Further, each cell differently uses the time slot resourcewithout inter-cell interference, so that each cell can assign many timeslot resources such that each cell receives multiple user terminals.According to such a method, since each cell transmits different data foreach of the user terminals depending on a time, a unicast user, abroadcast user, and a multicast user can be included within a singlecell. In order to communicate different user data within the cell withthe adjacent cell in such a communication environment without theinter-cell interference, the cell has to use the different time slotresource from that of the adjacent cell, such that it is possible toreduce the inter-cell interference.

Further, because the light sources included in each cell of FIG. 6 havedifferent cell IDs, when the PD included in each light source or the PDincluded in the cell receives a signal transmitted from the terminal, itis possible to recognize a position of the signal transmitted from theterminal by using the cell IDs.

FIG. 7 illustrates such a circumstance of the recognition of theposition of the signal transmitted from the terminal by using the cellIDs. When each of the light sources 515, 517, 519, 521, and 523 ismapped to a single cell 501, each of the light sources 515, 517, 519,521, and 523 is assigned a different cell ID, so that it is possible todiscriminate each of the light sources included within the identicalcell. The cell ID is assigned by an AP 513.

When the terminal U1 moves in a direction of the light sources 515, 517,and 519 included in the cell 501, the terminal U1 can move whilereceiving services from each of the light sources. When the terminal U1receives DL data while moving to a service area 503, 505, 507 of each ofthe light sources 515, 517, and 519 and then transmits an UL signal,i.e. ACK, NACK, or a response signal, in response to the received signalthrough the UL, the PD belonging to the area 503, 505, 507 of each ofthe light sources 515, 517, and 519, or a light source including thecorresponding PD, can receive the UL signal of the terminal. The AP 513can recognize a position of the terminal within the cell by using eachcell ID mapped to each of the light sources 515, 517, and 519. FIG. 8illustrates a case where the light sources included in each of the cellsuse identical cell IDs within each of the cells. A cell 601 is assignedCell 1 ID_1, a cell 603 is assigned Cell 2 ID_1, and a cell 605 isassigned Cell n ID_1 for use. Further, the terminal U1 included in thecell 601 uses a time slot vs n, the terminal U2 included in the cell 603uses a time slot vs n+1, and the terminal U3 included in the cell 605uses a time slot vs n+n for communication. An AP 607 is operated in theunit of a time slot resource like a structure of a frame 609.

FIG. 9 illustrates the construction of a system for supporting aterminal mobility, according to an embodiment of the present invention.It is assumed that cells 701, 705, and 709 are controlled by a single AP711 and the system is operated in the unit of a time slot resource likea structure of a frame 713. The frame 713 is divided into a DL and a UL,so that a terminal U1 715 can transmit/receive data in a time slotresource assigned to itself. Further, when each of the cells 701, 705,and 709 is assigned a time slot resource, i.e. a terminal U1 715 in thecell 701 is assigned a time slot vs n, a terminal U2 717 in the cell 705is assigned a time slot vs n+2, and a terminal U3 719 in the cell 709 isassigned a time slot vs n+n. The terminal U1 715 must be previouslyassigned a time slot resource to be used in an adjacent cell to move toduring the movement from the cell 701 to the cell 705 and the cell 709for the continuous communication. In order to secure the continuousmobility of the terminal U1 715, the AP 111 assigns a time slot resourceto the light sources (boundary light sources) positioned in a cellboundary area, such as boundary areas 703 and 707 of FIG. 9, so as tosupport the terminal mobility. By assigning the time slot resource,which is not used in the adjacent cell or in the cell including each ofthe light sources, to the boundary light sources positioned in the cellboundary areas 703 and 707, the terminal mobility can be secured. Whenthe terminal U1 715 is assigned the time slot vs n in the cell 701 andthen moves to the cell 705 during the data communication with the AP711, the U1 passes the boundary area 703. At this time, in order tocontinuously provide the data service, the terminal U1 has to beassigned the time slot resource available in the cell 705 to move fromthe AP 711 in the boundary area 703. The information on the timeresource assignment is informed to every terminal positioned within thecell by using the M (management) frame in the frame 713 or a controlframe that makes a notice of resource information. The time slotresource to be used in the terminal is transmitted to the terminal andthe terminal communicates the time slot resource assigned by the AP 711.

As described with reference to FIG. 9, since each of the light sourcesor the PD included in the light source area has the cell ID and theterminal ID is included in the received terminal signal, the terminaltransmits the ACK, the NACK, or the response signal in response to thedata received through the DL through the UL and the PD positioned ineach of the light sources or the PD positioned in the light source areareceives the terminal signal and transfers the received signal to the AP711, so that the AP 711 can recognize the movement direction of theterminal and the position of the light source. When the terminal isinitially set up, the AP recognizes the terminal and assigns the timeslot resource to each terminal, so that the AP 711 has been previouslyaware of the terminal ID. Therefore, when the terminal reaches theboundary area 703, the terminal receives the terminal UL signal in thecell boundary light source and transfers the received signal to the AP711, so that the AP 711 recognizes the fact that the terminal ispositioned in the cell boundary. In this respect, by using the time slotresource assigned to the terminal for the next DL frame, the AP 711informs the terminal of the fact that the terminal is positioned in thecell boundary through the cell boundary notice information and alsosimultaneously informs the terminal of information of the time slotresource in which interference with another user is not incurred in thecell to move to, so that the terminal is assigned the time slot resourceto be used in the next cell and simultaneously receives the continuousdata service from the previous cell. At this time, the terminalcommunicates with at least two time slot resources, so that the lightsource included in the cell boundary functions as a bridge resource forthe cell change of the terminal. By increasing or decreasing the lightsource area located in the cell boundary area, it is possible to controlthe mobility efficiency of the terminal according to the system.

The terminal U1 715 transmits/receives data with the time slot resourceassigned in the boundary area 703 in the cell 705 of FIG. 9, and movesfrom the cell 701 to the cell 705. The time slot resource recourseassigned to the terminal U1 715 in the cell 703 is one of the time slotresources, other than the time slot vs n+2 used by the terminal U2 717in the cell 705. In FIG. 9, the terminal U1 715 is assigned the timeslot vs n in the cell 701 and transmits/receives the data, and when theterminal U1 715 reaches the boundary area 703, the terminal U1 715 isassigned the time slot vs n+1 to be used in the next cell 705 togetherwith the cell boundary notice information, to perform the communication.From the reception moment of the cell boundary notice information, theterminal U1 715 transmits/receives the data by using the two time slotsvs n and vs n+1. Further, the terminal U1 715 uses the two time slotresources because it always receives the cell boundary notice when it ispositioned in the cell boundary area 703. However, the terminal U1 715uses different time resources from those of other terminals located inthe cells 701 and 705, so that it is possible to provide the continuousdata service without interference with the cell and other users.

When the terminal U1 715 is assigned the time slot vs n in the cell 701from the base station and the terminal U1 715 enters the boundary area703 during the transmission/reception of the data with the base station,the base station assigns the additional time slot vs n+1 to the terminalU1 715. The terminal U1 715 is assigned the additional time slot vs n+1,other than the time slot vs n+2 used by another terminal U2 717 in thecell 705, and the AP 711 provides the terminal U1 715 with the serviceby using the two time slots vs n and vs n+1, thereby securing theterminal mobility.

After the assignment of the time resource, the terminal responds to thetime resource assignment with confirm response information, the ACKsignal, etc. so that the interaction communication link for theassignment of the time slot resource between the base station and theterminal is established.

Further, the time when the terminal uses the two time slot resourcesbegins from the reception of the cell boundary notice information in theboundary area 703. When the terminal U1 715 leaves the boundary area 703and enters a stable region of the cell 705, the terminal U1 715 cannotreceive the cell boundary notice information, so that the terminal U1715 returns the time slot vs n used in the previous cell and uses therecently assigned time slot vs n+1 in the cell 705. The cell boundarynotice information used in the cell boundary informs the terminal of thecell boundary and is used for the purpose of addition or return of thetime resource assigned to the terminal together with the information ofthe time resource change. Additional time slot resource assignmentinformation, which is simultaneously transmitted with the cell boundarynotice information transmitted in the cell boundary area, can betransmitted with the cell boundary notice at the same time, or can beassigned within the cell after the reception of the notice information.The return of a specific time slot resource means that the communicationdoes not use the specific time slot resource. The specific time slotresource may be returned according to a request of the terminal.Further, according to the response of the terminal corresponding to thecorresponding time slot resource or a kind of light sources receivingthe response of the terminal corresponding to the corresponding timeslot resource, the return of the time slot resource in the AP can bedetermined.

After the terminal U1 715 moves from the cell 701 to the cell 705 viathe cell 703, when the cell boundary notice information disappears, theterminal U1 715 returns the time slot vs n assigned in the cell 701,thereby granting an opportunity to another user. Further, when theterminal U1 715 moves from the cell 705 to the cell 709, the same methodis performed. When the terminal U1 715 communicates with the basestation by using the time slot vs n+1 in the cell 705 and then moves tothe cell 709, the terminal U1 715 passes by the cell boundary 707. Atthis time, when the terminal U1 715 reaches the cell boundary area, thecell 709 is assigned a time slot vs n+3 that is one of not-used timeslot resources together with the cell boundary notice information, andthe terminal U1 715 continues the data service with the base station byusing the two time slots vs n+1 and vs n+3, as described in theaforementioned method.

Other user terminals U3 719 and U4 721 are assigned and use time slotsvs n+n and vs n+5, respectively, in the cell 709, so that an AP 711assigns the time slot vs n+3, which is one of time slot resources exceptfor the time slots vs n+n and vs n+5, to the terminal U1 715. Theterminal U1 715 transmits a response, the ACK signal, or the like, tothe AP 711, to establish the link. Further, when the terminal U1 715approaches the cell 709, the cell boundary notice information disappearsbecause the terminal U1 715 has left a boundary area 707 at which thecell boundary signal arrives. Therefore, the terminal U1 715 returns thepreviously used time slot vs n+1, thereby maximizing the time resourceutilization of another user. When the cells are separated from eachother, like the cell 701 and the cell 709, and the terminal userconsiders the assignment of the identical data, such as thebroadcasting, there is no interference between the time slot resources,so that it is possible to assign the identical time slot resource. Sucha process is performed in the AP 711 functioning as the base station.

FIG. 10 illustrates a case where a terminal U1 815 is received the cellboundary notice information and is assigned the time slot vs n+1 in acell boundary area 803 during the movement from a cell 801 to a cell 805and moves back to a stable region of the cell 801. The frame 813 isdivided into a DL and a UL, so that a terminal U1 815 cantransmit/receive data in a time slot resource assigned to itself.Further, when each of the cells 801, 805, and 809 is assigned a timeslot resource, i.e. a terminal U1 815 in the cell 801 is assigned a timeslot vs n, a terminal U2 818 in the cell 805 is assigned a time slot vsn+2, a terminal U3 819 in the cell 809 is assigned a time slot vs n+nand a terminal U4 821 in the cell 809 is assigned a time slot vs n+5.where a terminal U1 815 is received the cell boundary notice informationand is assigned the time slot vs n+3 in a cell boundary area 807.

In this case, when the terminal U1 815 leaves the boundary area 803, itfails to receive the cell boundary notice information, so that theterminal U1 815 returns the previously used time slot vs n assigned inthe cell 801 and communicates with an AP 811 in the cell 801 by usingthe time slot vs n+1 assigned in the boundary area 803. Therefore,according to the present invention, the AP 811 has to assign the timeslot resource, which is not used in the cells 801 and 805, to theterminal in the boundary area. Further, only when the adjacent cell inwhich the signal is overlapped does not use the identical time slotresource in the VLC, is it possible to communicate without interference.Therefore, when the terminal U1 815 receives the boundary noticeinformation and is assigned the time slot vs n+1 after entering theboundary area 803 by using the time slot vs n in the cell 801 and thenmoves back to the stable region of the cell 801, the cell boundarynotice information is not received, so that it is recognized that theterminal U1 815 enters the cell. Then, the terminal U1 815 returns thepreviously used time slot vs n and communicates by using the time slotvs n+1 assigned in the boundary area 803 in the cell 801 in which themovement of the terminal U1 815 starts, so that it is possible tocommunicate without interference between the adjacent cells.Specifically, the communication without interference between theadjacent cells can be achieved because the time slot resource assignedin the boundary area 803 is the time slot resource that is not used inthe adjacent cells. In the event of the broadcast mode, the usersincluded in each cell communicate with the identical time slot resource,so that if the time slot resource is not identical to that of theadjacent cell, there is no interference between the adjacent cells.Further, the identical time slot resources are assigned to theterminals, so that the terminals can communicate with the AP 811 withoutthe interference in the broadcasting mode.

Table 1 represents an example of frame information of the descriptionwith reference to FIGS. 9 and 10.

TABLE 1 Management payload field bit usage Src_multi_info n sourcemultiple channel resource assignment Des_multi_info n destinationmultiple channel resource assignment H_pattern n multiple channelhopping VF_info n visibility frame notification for resource anddestination Src_mode n source notifications G_cell_ID n granular cellassignment Fractional_Src n frame resource notifications Mode_type nmulticast, broadcast, unicast S_Release_slot n start and release slotfor broadcast special mobility n request special mobility MS_ID nterminal ID or user ID # of time slot n number of time slot resourceresource Cell_info n Transmit Cell ID(PD_ID) or AP ID B_info n cellboundary information S_info n virtual time slot resource assignment

In order to operate the VLC system as illustrated in FIGS. 9 and 10, theVLC system inserts cell ID information Cell_ID_info, cell boundarynotice information B_info, and time slot resource assignment informationS_info in the M frame as represented in Table 1, and transmits theinformation to the corresponding terminal. With regard to the cellboundary notice information B_info, the VLC system can include variousconstructions of the cells and there is a part in which it is difficultto inform the terminals of the cell or the cell change information, sothat the cell boundary notice information B_info is transmitted to theterminal from the AP and informs the terminal of the cell range.Therefore, the terminal receives the cell boundary notice informationB_info, so that the terminal can recognize that the terminal is locatedin the cell boundary and thus it is necessary to change the time slotresource according to the movement to the adjacent cell. Further, theterminal can recognize the cell to which the terminal belongs or the APof the base station through the cell ID information Cell_ID_info. Theterminal can respond to the received cell boundary notice informationB_info with the ID of each terminal or ACK, a general reception responsesignal, etc. through the UL. The time slot resource assignmentinformation S_info represents the time slot resource assigned for use inthe cell, to which the terminal will move, together with the cellboundary notice information B_info that is been received in the cellboundary. Therefore, by using the time slot resource, the terminal isassigned the time slot resource that is not been used in the currentcell or the cell to which the terminal will move, so that the terminalcan continue the data service with the base station.

The cell ID information Cell_ID_info, the cell boundary noticeinformation B_info, and the time slot assignment information S_info areapplied to the user or the terminal included in application terminalinformation MS_ID included in the frame.

FIG. 11 illustrates an example of the VLC frame capable of includingTable 1.

FIG. 11 illustrates a super frame format, according to anotherembodiment of the present invention. Referring to FIG. 11, a VLC frame900 includes a beacon field 910, a management field 920 includingcontrol information for the VLC, and a data frame 930 including data.

The beacon field 910 includes a preamble 911, a header 912, a HeaderCheck Sequence (HCS) 913, a Protocol Data Unit (PDU) 914, and a FrameCheck Sequence (FCS) 915. The preamble 911 includes informationdetermining if the VLC frame 900 communicates using multiple time slotresources. A reception side receiving the VLC frame 900 examines thepreamble 911 of the beacon field 910 upon the reception of the frame 900and interprets the frame 900 in a form of a super frame 1000 when theVLC frame 900 does not use the time slot resource.

The management field 920 includes a preamble 921, a header 922, an HCS923, a MAC header 924, a PDU 925, and a FCS 926.

The data frame 930 includes a single contention slot 931, apredetermined number of DL time slot resources 932, and a predeterminednumber of UL time slot resources 933. The DL time slot resources 932 andthe UL time slot resources 933 are individually assigned to multipleusers when the communication is operated in the broadcast mode.

The single DL time slot resource 932 and UL time slot resource 933include a preamble 941, a header 942, an HCS 943, and a data field 944.The data field 944 includes a pair including a single MAC header 951 andmultiple PDUs 952 and FCSs 953.

The super frame 1000 is the VLC frame in the unicast communication mode,according to the embodiment of the present invention, and includes apreamble 1101, a header 1102, an HCS 1103, and a data field 1105. Thedata field 1004 includes a pair including a single MC header 1111 andmultiple FDUs 1112 and FCSs 1113.

Table 1 can be included in the M field 920.

Further, according to another embodiment of the present invention, a PDpayload or the header can include the cell ID information Cell_ID_info,the cell boundary notice information B_info, and the time slot resourceassignment information S_info.

FIG. 12 illustrates the security of the mobility generated when theterminal U1 passes by the cell adjacent area (a boundary area 1009,1011,1013 and 1015 and an overlapped boundary area 1023) before movingfrom a cell 1003 to any one of a cell 1001, a cell 1005, and a cell1007.

When the terminal U1 moves from the cell 1003 to the cell 1001 (in adirection 1017) and from the cell 1003 to the cell 1007 (in a direction1019), the method illustrated in FIGS. 9 and 10 is applied. However,when the U1 moves from the cell 1003 to the cell 1005 (in a direction1021), it must pass by an overlapped boundary area 1023 in which all ofthe cell 1001, the cell 1003, and the cell 1007 are adjacent to eachother. Therefore, since the AP has been aware of the light sourcelocated in the boundary of each of the cell 1001, the cell 1003, thecell 1005, and the cell 1007 and the ID of the cell including the PD ofthe light source, the light source, i.e. the PD, in the boundaryoverlapped area assigns a time slot vs n+9 that is not used in the cell1001, the cell 1003, the cell 1005, and the cell 1007. This reduces thecommunication interference in the overlapped light source area. Thedefinition of the overlapped area size can be variously determinedaccording to the system, as similar with FIGS. 9 and 10 in which thesize of the boundary area for the overlapped area is varied according tothe system condition.

When the terminal U1 included in the cell 1003 arrives at the overlappedboundary area 1023 during the movement to the cell 1005, the terminal U1receives the cell boundary notice information and is assigned the timeslot vs n+9, which is not used in the cell, in the overlapped boundaryarea 1023 and communicates with the AP of the base station with the twotime slots vs n and vs n+9. When the terminal U1 leaves the overlappedboundary area 1023 and reaches the cell 1005, the terminal U1 cannotreceive the cell boundary notice information, so that the terminal U1returns the time slot vs n, which had been used in the previous cell1003, and communicates with the base station with the time slot vs n+2in the cell 1005, to continuously communicate.

Further, when the terminal U1 is located in the overlapped boundary area1023, the movement of the terminal U1 to the cell 1001, the cell 1005,or the cell 1007 is determined by the user of the terminal U1. In thisrespect, the terminal U1 is assigned the time slot resource that is notused by users U2, U3, U4, and U5 included in the adjacent cell, so thatit is possible to communicate without the interference. In FIG. 12, eachof the light sources included in the cell 1001, the cell 1003, the cell1005, and the cell 1007 has a cell ID for discrimination, so that it ispossible to discriminate the light source or the PD within each cell.Specifically, it is possible to identify the position of the lightsource or the PD to which the terminal belongs through the cell ID.

FIG. 13 illustrates another method of returning the time slot resource,which is a resource used in the previous cell, in the cell to which theterminal will move. According to the return method of the time slotresource of FIG. 10, when the terminal U1 moves toward the cell 805during the communication with the AP by using the time slot vs nassigned in the cell 801, the terminal U1 has to pass by the boundaryarea 803. In this case, when the terminal U1 approaches the boundaryarea 803, the terminal U1 receives the cell boundary notice informationand is assigned the time slot vs n+1 and then enters the stable regionof the cell 805. Thereafter, when the terminal U1 fails to receive thecell boundary notice information transmitted from the boundary area 803,the terminal U1 returns the time slot vs n used in the previous cell801, thereby granting an opportunity of the time resource assignment toanother user. In this method, it is described the method of returningthe previous time slot resource and the assigned time slot resourcebased on the cases where the terminal U1 receives and does not receivethe cell boundary notice information in the cell boundary. FIG. 13illustrates another method of returning the time resource.

In FIG. 13, cell_n 1139 includes light sources 1117, 1119 and 1121,cell_m 1141 includes light sources 1123, 1125, 1127 and 1129, and cell_p1143 includes light sources 1131, 1133 and 1135.

When the terminal U1 moves to the cell and approaches a light source1121 included in the cell boundary area during the communication with anAP 1137 by using the time slot vs n assigned from light sources 1117 and1119, the light source 1121 transmits the cell boundary noticeinformation to the terminal U1. The light source 1121 has an ID Cell_nID n+3. Specifically, the light source n+3 1121 included in a cellCell_n 1139 transmits the cell boundary notice information to theterminal U1 and the AP 1137 assigns the time slot vs n+1 to the terminalU1. When the terminal U1 continuously moves and approaches a lightsource 1123 included in the cell boundary area, the light source 1123transmits the cell boundary notice information by using Cell_m ID M+1,i.e. a light source m+1 1123 included in a cell Cell_m 1141. Therefore,the terminal U1 recognizes that it is included in a different cell byusing the light source cell ID because of the different light source IDwhich transmits the cell boundary notice information, so that theterminal U1 returns the previously used time slot vs n to the basestation. Therefore, the terminal U1 communicates with the AP 1137 byusing the assigned time slot vs n+1. Further, even if the terminal U5included in the light source 1131 of a cell 1143 is assigned the timeslot vs n identical to that assigned to the terminal U1 in the cell Celln 1139, it is possible to communicate without the interference becausethe cells are isolated from each other so as to prevent the cells frombeing overlapped. By using this method, the VLC reuses the time slotresource, thereby improving the entire capacity of the system. FIG. 14illustrates a method of securing the mobility in a case where the lightsources included in each of cells 1201, 1203, and 1205 have a singlecell ID in a unit of the cell, not independent cell IDs. An AP 1211 usesframe 1213. On an assumption that the terminal U1 moves from the cell1201 to the cell 1203, the light sources included in each of the cells1201 and 1203 have a cell ID Cell ID 1 and Cell ID 2 in the unit of thecell, so that the terminal U1 cannot identify the position of themovement within the cell with the light source or the light sourceincluding the PD. However, because a light source included in a boundaryarea 1207 has a cell ID Cell ID n, when the terminal U1 approaches thecell boundary area 1207 during the movement from the cell 1201 to thecell 1203, the terminal U1 transmits a signal of the UL response withrespect to the DL resource to the light source or the PD included in thelight source, so that an AP 1211 recognizes that the terminal U1 islocated in the cell boundary. Further, the light source included in thecell boundary area 1207 transmits the cell boundary notice informationaccording to the approach of the terminal U1, so that the terminalrecognizes that it is located in the cell boundary. Further, theterminal U1 is assigned the time slot vs n+1 assigned by the AP 1211 andcommunicates with the AP 1211 with the time slots vs n and vs n+1.Further, when the terminal U1 fails to receive the boundary noticeinformation transmitted from the boundary area 1207 or receives a DLsignal transmitted from the cell 1203 by using the time slot vs n+1 inapproaching the cell 1203, the terminal U1 returns the time slot vs nassigned in the cell 1201. FIG. 14 illustrates the method of supportingthe mobility when the light sources included in the single cellcommunicate with the identical cell ID.

FIG. 15 illustrates the security of the mobility generated when theterminal U1 passes by the cell adjacent area (a boundary area 1309,1311,1313 and 1015 and an overlapped boundary area 1323) before movingfrom a cell 1303 to any one of a cell 1301, a cell 1305, and a cell1307. FIG. 15 illustrates a method of supporting the mobility in avisible light cell environment in which a single cell ID is granted toeach cell when the terminal U1 is located in the overlapped boundaryarea as illustrated in FIG. 12. On the assumption that the terminal U1moves from a cell 1303 to a cell 1305 (in a direction 1321), theterminal U1 passes by an overlapped boundary area 1323.

In the overlapped boundary area 1323 in which each of the cells isoverlapped with each other, the light source or the PD included in thelight source uses a cell ID Cell ID 9, and when the terminal U1approaches the overlapped boundary area 1323, the light source or the PDincluded in the light source assigns a time slot vs n+9 by using a cellID Cell ID 9. Since the light source or the PD included in the lightsource has the cell ID Cell ID 9, when the terminal U1 is located in theoverlapped boundary area 1323, it is possible to recognize the positionof the terminal U1 with the terminal UL signal. Therefore, when theterminal U1 is located in the overlapped boundary area 1323, theterminal U1 receives the cell boundary notice information and isassigned the time slot vs n+9 and uses the time slots vs n and vs n+9 inthe overlapped boundary area 1323. Further, when the terminal U1 entersthe cell 1305 and does not receive the cell boundary notice informationfrom the overlapped boundary area 1323, the terminal U1 returns the timeslot vs n assigned in the cell 1303. Therefore, the terminal U1communicates using the time slot vs n+9 in the cell 1305. The method forsecuring the mobility with respect to a movement direction 1317 and 1319of the terminal U1 from the cell 1303 is identical to that illustratedin FIGS. 9, 10, and 12. FIGS. 16A and 16B illustrate an operation of theVLC terminal, according to an embodiment of the present invention. Asillustrated in FIGS. 16A and 16B, in step 1403, the VLC terminalinitially accesses a light source of a cell to which the terminalbelongs. In step 1405, the VLC terminal makes synchronization with thebase station AP through the light source that provides the area of theVLC terminal with the VLC service. In step 1407, the terminal identifiesan assigned time slot vs n, and in step 1409, the terminal communicateswith a corresponding light source by using the assigned time slot vs n.When the terminal moves to the cell in step 1411, the terminalidentifies if the cell boundary notice information transmitted from thelight source located in the boundary of the cell is received in step1413. When the boundary notice information is not received, the terminalcontinues the service with the light source of the current cell in step1415. However, when the boundary notice information is received, theterminal identifies the time slot vs n+1 assigned in the cell boundaryin step 1417. The terminal transmits a response according to theassigned time slot resource in step 1419 and the terminal communicatesusing the two time slots vs n and vs n+1 in step 1421. Then, theterminal identifies an ID of the light source transmitting the cellboundary notice information in step 1423, to proceed to step 1425. TheVLC terminal identifies if the ID of the light source transmitting thecell boundary notice information is changed in step 1425. When the lightsource ID is not changed in step 1425, the terminal continues thecommunication by using the two time slot resources in the cell boundaryin step 1427. When it is identified that the light source ID is changedin step 1425, the terminal returns the used time slot vs n in step 1429,and continues the communication in the next cell to move to by using thetime slot vs n+1 in step 1431.

According to another embodiment of the present invention, the VLCterminal does not return the time slot resource according to the changeof the ID of the light source transmitting the cell boundary noticeinformation, but determines the return of the time slot resource basedon the reception of the cell boundary notice information, the process ofwhich corresponds to steps 1433, 1435, and 1437.

Specifically, the terminal identifies the ID of the light sourcetransmitting the boundary notice information in step 1423 and proceedsto step 1433. The terminal identifies if the cell boundary noticeinformation is continuously received in step 1433. When the cellboundary notice information is continuously received, the terminalcontinues the communication by using the two time slot resources in thecell boundary in step 1427. However, when the cell boundary noticeinformation is not received, the terminal maintains the recentlyassigned time slot resource among the currently used time slot resourcesand returns the previously assigned time slot vs n in step 1435. Then,the terminal continues the communication using the recently assignedtime slot vs n+1 in the cell to which the terminal will move in step1437.

FIGS. 17A and 17B illustrate an operation of the base station AP,according to an embodiment of the present invention. When the basestation receives an initial access request from the terminal located ina service area of a certain light source in step 1503, the base stationidentifies a cell including the light source and makes synchronizationwith the corresponding terminal through the light source in step 1505.The base station searches for a time slot resource to be assigned to theterminal in step 1507, and assigns the time slot vs n to the terminal instep 1509. Then, the base station communicates with the terminal byusing the time slot vs n in step 1511, and detects ACK or a responsetransmission signal in response to the data reception from the terminalin step 1513. The terminal identifies if the ACK or the response signalis detected in the light source located in a boundary area of thecorresponding cell in step 1515. When the ACK or the response signal isnot detected, the terminal continues the service by using the currentlyassigned time slot vs n in step 1517.

However, when the ACK or the response signal of the terminal is receivedthrough the light source located in the boundary area of thecorresponding cell in step 1515, the base station transmits the cellboundary notice information to the terminal in step 1519. The basestation searches for a time slot resource currently used or scheduled tobe used in the cell to which the terminal may move in step 1521. Thecell, to which the terminal will move, refers to a cell adjacent to thecell in which the terminal is currently located. The base stationassigns a single time slot vs n+1 among the assignable time slotresources, except for the time slot resource searched in step 1521, tothe terminal in step 1523. After the assignment of new time slotresource to the terminal, the base station identifies if a responsesignal in response to the time slot resource assignment is received instep 1525.

Then, the base station identifies if a signal of the terminal isreceived through the light source located in the boundary area of thecell prior to the movement of the terminal in step 1527. When the signalof the terminal is received, the base station continuously transmits thecell boundary notice information to the terminal in step 1529. When thesignal of the terminal is not received through the light source locatedin the boundary area of the cell prior to the movement of the terminal,the base station receives the transmission signal of the terminal andidentifies if the light source transmitting the transmission signalcorresponds to the light source located in the stable region of theprevious cell in step 1531. The stable region refers to a region exceptfor the cell boundary area in the cell. When the signal of the terminalis received through the light source located in the stable region of thecell, the base station identifies the return of the time slot vs n instep 1533.

However, when the signal of the terminal is not received through thelight source located in the stable region of the cell, the base stationidentifies if the signal of the terminal is transmitted through thelight source located in the boundary area of the cell to which theterminal will move in step 1535. When the signal of the terminal istransmitted through the light source located in the boundary area of thecell, the base station continuously transmits the cell boundary noticeinformation to the terminal in step 1537. Then, the base stationidentifies if the time slot vs n is returned in step 1539, and registersthe terminal to the cell to which the terminal moves in step 1541. Next,the base station assigns the time slot resource of which the return hasbeen identified in step 1539 to another terminal for use in step 1543.

When the signal of the terminal is not transmitted through the lightsource located in the boundary area of the cell to which the terminalwill move in step 1535, the base station continuously identifies throughwhich light source the signal of the terminal is received in step 1545.When it is identified that the terminal is located in the cell to moveto in step 1547, it proceeds to step 1539. However, when it isidentified that the terminal is not located in the cell to move to instep 1547, the base station continuously identifies a relevant lightsource.

Although the above embodiment is based on a case where a specific ID isdesignated to each light source including at least one LED and at leastone PD, it is possible, according to another embodiment of the presentinvention, to separately indicate an LED and a PD included in a singlelight source by different IDs, respectively. Otherwise, each LED or eachPD may be individually indicated by a specific ID.

FIGS. 18 and 19 illustrate the movement of the terminal between adjacentcells according to another embodiment of the present invention.

On an assumption that a boundary area to which a cell j 2201 and a cellk 2205 are adjacent is a boundary area 2203, when a terminal DS1 2207moves from the cell j 2201 to the cell k 2205 in directions 2209 and2211, the terminal DS1 2207 has to pass by the boundary area 2203 towhich the cell j 2201 and the cell k 2205 are adjacent. Each of thecells includes multiple light sources functioning as the transmissionunit and multiple PDs functioning as the reception unit. Each of thelight sources and the PDs has the individual ID, so that the ID of eachof the light sources and the PDs is used for the time resourceassignment for the mobility. It is possible to make a single ID throughcombining the ID of each of the light sources and the ID of each of thePDs. Further, data or a control signal is transmitted/received betweenthe terminal DS belonging to the cell connected to the AP 2219 and theAP 2219 by using the each of the light sources and the PDs. When theterminal DS1 2207 located in the cell j 2201 moves to the cell k 2205,the terminal DS1 2207 passes by the boundary area 2203 included in eachof the cells. When the terminal DS1 2207 is assigned a time slot ts n inthe cell j 2201 from the AP 2219 and the terminal DS1 2207 communicateswith the AP 2219 by using the time slot ts n, every light sourceincluded in the cell j 2201 in which the terminal DS1 2207 is locatedtransmits the DL data or the control signal transmitted from the AP 2219to the terminal DS1 2207 during the time slot ts n assigned to theterminal DS1 2207. However, there is a high probability that theterminal DS1 2207 actually receives the DL data or the control signaltransmitted from the light source that is the closest to the cell j2201, so that the terminal DS1 2207 receives the DL data or the controlsignal from the cell light source Cell j ID_7 in the cell j 2201.Therefore, the terminal DS1 2207 detects the light source ID from the DLdata or the control signal and recognizes the ID of the light sourcethat currently provides the service in the cell j 2201. Further, an APID is transmitted together with the UL data or the control signal, sothat the terminal DS1 2207 can discriminate the AP by using the AP ID.When the terminal DS1 2207 transmits the UL signal in response to the DLsignal, the terminal DS1 2207 transmits the UL signal by using theassigned time slot ts n. In this case, when the terminal DS1 2207 doesnot move in the cell j 2201, the terminal DS1 2207 transmits the ULsignal in the area of the light source Cell j ID_7 receiving the DLsignal and the UL signal is received in the PID 1_7, so that the AP 2219controlling the PID 1_7 can be aware that the current terminal DS1 2207is located in the PID 1_7 within the cell j 2201. Therefore, the AP 2219and the terminal DS1 2207 can recognize the AP ID transmitted throughthe DL and the transmission light source ID, respectively, and the APcan identify the position within the cell of the terminal DS1 2207transmitted through the UL by using the PID 1_7.

When the terminal DS1 2207 moves and approaches the boundary area 2203,the light sources included in the cell j 2201 DL/UL communicate with theterminal DS1 2207 by using the time slot ts n, so that the light sourceCell j ID_8 provides the terminal DS1 2207 with the DL service in thecell j 2201 during the time slot ts n. Therefore, even if the terminalDS1 2207 moves to the cell boundary area 2203, the terminal DS1 2207continuously receives the DL service. Further, the terminal DS1 2207receives the DL service by using the light source ID Cell j ID_8 afterthe movement, so that the terminal DS1 2207 can recognize that the lightsource ID is changed. That is, the terminal DS1 2207 can recognize thatthe light source ID is changed from Cell j ID_7 to Cell j ID_8 throughthe DL. Further, when the terminal DS1 2207 approaches the boundary area2203 in the direction 2209, the AP 2219 can recognize the movement ofthe terminal DS1 2207 by using the change of the PID, which is receivedduring the transmission of the UL by the terminal DS1 2207, from PID 1_7to PID 1_8. Therefore, when the terminal DS1 2207 enters the boundaryarea 2203, the AP 2219 informs the boundary information and additionallyassigns a time slot ts n+2 to be used in the cell 2205 to the terminalDS1 2207 according to the movement of the terminal DS1 2207 in thedirection 2211. The additionally assigned time slot resource is the timeslot resource which is not used by a terminal DS2 located in the cell2201 and the cell 2205.

In FIG. 18, the terminal DS1 2207 transmits/receives data with the lightsource and the PDS connected with the AP 2219 by using the time slot tsn being used in the cell j 2201. Further, when the terminal DS1 2207moves in the direction 2209 and approaches the boundary area 2203, theAP 2219 assigns the additional time slot ts n+2 to the terminal DS1 2207by using the time slot ts n together with the boundary information.

The PID 1_8 receives the UL transmitted by the terminal DS1 2207 and theAP 2219 transmits the boundary information by using Cell j ID_8 becausethe terminal DS1 2207 is located in the boundary area 2203. At thistime, the terminal DS1 2207 is assigned the time slot ts n+2 from thelight source Cell j ID_8 included in the boundary area 2203. Theboundary information can be transmitted through each of the lightsources, Cell j ID_4, Cell j ID_8, Cell j ID_12, Cell k ID_1, Cell kID_5, and Cell k ID_9, or the time slot ts n+2 can be assigned throughonly the light source Cell j ID_8 in which the terminal DS1 2207 islocated.

The time slot resources currently assigned to the terminal DS1 2207 arets n and ts n+2. In order to return one of the assigned time slotresources to the AP 2219, the terminal DS1 2207 may determine the changeof the light source ID information according to a movement of theterminal DS1 2207, which is transmitted in the DL, when the terminal DS12207 located in the boundary area 2203 moves from the Cell J ID_8 toCell k ID_5. Therefore, when the light source ID through the DL changes,the terminal DS1 2207 makes a request for a return of the previouslyused time slot ts n among the currently assigned time slot resources tothe AP 2219 through the UL. According to another method of returning thetime slot resource, when the terminal DS1 2207 passes by the boundaryarea 2203 and moves to the cell k 2205, the terminal DS1 2207 fails toreceive the boundary information transmitted from the boundary area2203, so that the terminal DS1 2207 makes a request for the return ofthe previously assigned time slot ts n among the two assigned time slotresources to the AP 2219 through the UL. In this process, when theterminal DS1 2207 receives the response confirmation with respect to thereturn of the time resource from the AP 2219, the terminal DS1 2207UL/DL communicates with the AP 2219 by using only the time slot ts n+2.A frame 2217 represents a TDD (Time Division Duplex) frame for theassigned time slot resources. The VLC frame 2217 includes the DL throughwhich the AP 2219 transmits the data and control information by usingthe light source and the UL through which the terminal DS1 transmits thedata and control information to the AP 2219 by using the PID.

Further, in FIG. 18, when the terminal DS1 2207 receives the DL signalfrom the light source Cell j ID_8 in the boundary area 2203 and thenmoves to the light source Cell j ID_4 or Cell j ID_12, the terminal DS12207 comes to know that the DL light source ID is changed from the Cellj ID_8 to Cell j ID_4 or Cell j ID_12, so that the terminal DS1 2207 canmake a request for the return of the previously assigned time slot ts nbetween the time slots ts n and ts n+2 assigned from the AP 2219 to theAP 2219. According to another method of returning the time slotresource, as soon as the terminal DS1 2207 is additionally assigned thetime slot ts n+2 from the AP 2219, the terminal DS1 2207 operates atimer and then makes a request for the return of the previously assignedtime slot ts n to the AP 2219 through the UL after a preset time.According to another method of returning the time slot resource, whenthe light source ID transmitting the cell boundary information throughthe DL, the terminal can make a request for the return of the previouslyassigned ts n between the time slots ts n and ts n+2 assigned from theAP 2219 to the AP 2219 through the UL. According to another method ofthe time resource return, when the terminal DS1 2207 enters the cell k2205 from the cell k boundary and fails to receives the cell k boundaryinformation, the terminal DS1 2207 makes a request for the return of thepreviously assigned ts n between the time slots ts n and ts n+2 assignedfrom the AP 2219 to the AP 2219 through the UL. FIG. 19 illustrates theprocess of the return of the time slot resource when the terminal DS12207 is assigned the time slot ts n+2 from the AP 2219 in the cell jboundary 2403 and moves back to the cell j 2401.

The process in which the DS1 2407 approaches the cell j boundary 2403and is assigned the time slot ts n+2 is identical to that of FIG. 18.However, in FIG. 19, in the event that the terminal DS1 2407 has had theadditionally assigned time slot ts n+2 when it moves back to the cell j2401 in a direction 2409, the terminal DS1 2407 returns the time slotresource between the time slots ts n and ts n+2 to the AP 2219. When theterminal DS1 2407 moves back to the cell j 2401 and fails to receive thecell j boundary information transmitted from the cell boundary area2403, the terminal DS1 2407 transmits a request for the return of thepreviously assigned time slot ts n to the AP through the UL. When thelight source Cell j ID_8 transmits the cell j boundary information andassigns the time slot ts n+2 to the terminal DS1 2407, and then theterminal DS1 2407 moves to the light source Cell j ID_7 and fails toreceive the cell j boundary information, the terminal DS1 2407 transmitsa request for the return of the previously used time slot ts n to the AP2219 to the PID 1_7. Further, the AP 2219 transmits a response accordingto the request for the return of the time slot ts n of the terminal DS12407 received in the PID 1_7 by using the light source Cell j ID_7through the DL.

Table 2 illustrates an example of the frame information of thedescription of FIGS. 18 and 19. Referring to Table 2, it can be notedthat a DS_ID field includes a Device ID and a Cell_info field includesan Optical resource ID.

TABLE 2 Management Down/Up payload filed bit Usage/Description LinkSrc_multi_info 7 Available channels information in D/LCoordinator(Access Point) ex: 0000000 No multiple channel mode ex:0000001 using channel “A” ex: 0000101 using channel “A” and “C”Des_multi_info 7 Available channels information in mobile U/L device ex:0000000 No multiple channel mode ex: 0000001 using channel “A” ex:0000101 using channel “A” and “C” H_pattern 5 Channels hoppinginformation D/L VF_info_type 1 Using Visible Frame transmission D/L ex1: Receiver transmit the Visible Frame ex 0: Receiver Does Not transmitthe Visible Frame G_cell_ID 10 Granular cell size D/L Fractional_Src 2Using fractional resource assignment D/L ex 1: yes(fractional resourceassignment) ex 0: no (general resource assignment) Mode type 2 Mode typeD/L ex 1: multicast S_Release_slot 5 The start slot and release slotinformation for D/L broadcast mode Spatial mobility 2 Using Spatialmobility D/L DS_ID 10 Device ID D/L, U/L Number of Time 5 Assigned timeslot number D/L slot Cell_info 10 Cell ID, Optical Source ID, AP ID(D/L)D/L, U/L PD_ID(U/L) ex: Celln_ID_n B_info 2 Cell boundary informationD/L S_info 5 Time slot assignment and response(D/L) D/L, U/LRequest(U/L)

While the present invention has been shown and described with referenceto certain embodiments and drawings thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A visible light communication method in a visiblelight coordinator, which discriminates between at least onecommunication area cell that includes at least one light source and aboundary area positioned between the communication area cell and anothercommunication area cell adjacent to the communication cell area, andwhich provides time division visible light communication, the methodcomprising the steps of: detecting a position of a visible lightcommunication terminal; determining a first communication area cellcorresponding to the position of the detected visible lightcommunication terminal; determining a first time slot for receiving dataof the visible light communication terminal in the first communicationarea cell; assigning a second time slot, which has not been assigned tothe visible light communication terminal, to the visible lightcommunication terminal, when the visible light communication terminal islocated in a boundary area of the first communication area cell; andcommunicating with the visible light communication terminal using thesecond time slot.
 2. The method as claimed in claim 1, wherein detectingthe position comprises: identifying a light source receiving datatransmitted from the visible light communication terminal; anddetermining that the visible light communication terminal is located inthe boundary area when the light source corresponds to a light sourceincluded the boundary area, and determining that the visible lightcommunication terminal is located in a stable region when the lightsource corresponds to a light source included in the stable region ofthe first communication area cell, except for the boundary area.
 3. Themethod as claimed in claim 1, wherein the second time slot is a timeslot that is not used in the first communication area cell and a secondcommunication area cell adjacent to the first communication area cell.4. The method as claimed in claim 2, wherein in communicating with thevisible light communication terminal, the visible light communicationterminal communicates using the first time slot and the second timeslot.
 5. The method as claimed in claim 4, further comprising stoppingthe communication with the visible light communication terminal usingthe first time slot, when a response corresponding to the second timeslot is received from the visible light communication terminal.
 6. Themethod as claimed in claim 4, further comprising stopping thecommunication with the visible light communication terminal using thefirst time slot, when the light source receiving the data transmittedfrom the visible light communication terminal corresponds to the lightsource included in the stable region.
 7. The method as claimed in claim4, further comprising stopping the communication with the visible lightcommunication terminal using the first time slot, when the light sourcereceiving the data transmitted from the visible light communicationterminal corresponds to the light source included in the secondcommunication area cell.
 8. The method as claimed in claim 7, wherein anID of the light source receiving the data transmitted from the visiblelight communication terminal is an ID of the adjacent communication arecell.
 9. The method as claimed in claim 4, further comprising stoppingthe communication with the visible light communication terminal usingthe first time slot, when the light source receiving the datatransmitted from the visible light communication terminal corresponds tothe light source included in the stable region of the secondcommunication area cell.
 10. The method as claimed in claim 4, furthercomprising stopping the communication with the visible lightcommunication terminal using the first time slot, when the second timeslot has been assigned and then a predetermined time passes.
 11. Themethod as claimed in claim 1, wherein boundary area notice informationis transmitted through the light source included in the boundary area.12. A visible light coordinator, which discriminates between at leastone communication area cell that includes at least one light source anda boundary area positioned between the communication area cell andanother communication area cell adjacent to the communication area cell,and which provides time division visible light communication, thevisible light coordinator comprising: a memory for storing informationof the at least one communication area cell; and a controller fordetecting a position of a visible light communication terminal,determining a first communication area cell corresponding to theposition of the detected visible light communication terminal,determining a first time slot for receiving data of the visible lightcommunication terminal in the first communication area cell, assigning asecond time slot, which has not been assigned to the visible lightcommunication terminal, to the visible light communication terminal whenthe visible light communication terminal is located in a boundary areaof the first communication area cell, and communicating with the visiblelight communication terminal by using the second time slot.
 13. Thevisible light coordinator as claimed in claim 12, wherein thecontroller, in order to detect the position of the visible lightcommunication terminal, identifies a light source receiving datatransmitted from the visible light communication terminal, determinesthat the visible light communication terminal is located in the boundaryarea when the light source corresponds to a light source included in theboundary area, and determines that the visible light communicationterminal is located in a stable region when the light source correspondsto a light source included in the stable region, which is a region ofthe first communication area cell, except for the boundary area.
 14. Thevisible light coordinator as claimed in claim 12, wherein the secondtime slot is a time slot that is not used in the first communicationarea cell and a second communication area cell adjacent to the firstcommunication area cell.
 15. The visible light coordinator as claimed inclaim 13, wherein the controller communicates with the visible lightcommunication terminal using the first time slot and the second timeslot.
 16. The visible light coordinator as claimed in claim 13, whereinthe controller stops the communication with the visible lightcommunication terminal using the first time slot when a responsecorresponding to the second time slot is received from the visible lightcommunication terminal.
 17. The visible light coordinator as claimed inclaim 13, wherein the controller stops the communication with thevisible light communication terminal using the first time slot when thelight source receiving the data transmitted from the visible lightcommunication terminal corresponds to the light source included in thestable region.
 18. The visible light coordinator as claimed in claim 13,wherein the controller stops the communication with the visible lightcommunication terminal using the first time slot when the light sourcereceiving the data transmitted from the visible light communicationterminal corresponds to the light source included in the secondcommunication area cell.
 19. The visible light coordinator as claimed inclaim 13, wherein the controller stops the communication with thevisible light communication terminal using the first time slot when thelight source receiving the data transmitted from the visible lightcommunication terminal corresponds to the light source included in thestable region of the second communication area cell.
 20. The visiblelight coordinator as claimed in claim 13, wherein the controller stopsthe communication with the visible light communication terminal usingthe first time slot when the second time slot has been assigned and thena predetermined time passes.
 21. The visible light coordinator asclaimed in claim 12, wherein boundary area notice information istransmitted through the light source included in the boundary area.